Captain Hardt, a World War I German U-boat commander, is ordered to lead a mission to attack the British Fleet at Scapa Flow. He sneaks ashore on the Orkney Islands to meet his contact, Fräulein Tiel. Tiel has taken over the identity of local schoolteacher, Miss Anne Burnett, who had been kidnapped by German agents. Hardt finds himself attracted to her, but Tiel shows no interest. The Germans are aided by a disgraced Royal Navy officer, the former Commander Ashington, who, according to Tiel, has agreed to aid the Germans after losing his command due to drunkenness. Tiel implies that she has slept with Ashington to obtain his cooperation. Now equipped with the crucial information he needs about the fleet movements, Hardt makes rendezvous with his submarine to arrange the attack. Returning to the house, and confident that all is going to plan, Hardt make advances to Tiel, but she rebuffs him. She leaves the house, believing she has locked Hardt in, but he gets out and secretly follows her, discovering that she has gone out to meet Ashington. Hardt overhears them talking and learns the truth - the British are fully aware of his presence and his mission, and his 'contacts' are really British double-agents.